Process for controlling path in situ combustion front



Mar h 7, 19 H. w. PARKER 2,973,813

PROCESS FOR CONTROLLING PATH OF IN SITU COMBUSTION FRONT Filed Sept. 15, 1958 AIR OR PROD PREMIXED FUEL & AIR

SUBSTRATUM INVENTOR.

H. W. PARKER BYw/gw A TTORNE Y5 United States Patent" '0" PROCESS FOR CONTROLLING PATH INSITU COMBUSTION FRONT Harry W. Parker, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Sept. 15, 1958, Ser. No. 761,070 Claims. 01. 166-11) This invention relates to the recovery of componentsof a carbonaceous stratum by insitu combustion. A specific aspectof the process pertains to controlling the path of an inverse in situ combustion front in a relatively thick carbonaceous stratum.

In situ combustion in the recovery of hydrocarbons from underground strata containing carbonaceousmaterial is becoming more prevalent in the petroleum industry. In this technique of production, combustion is initiated in the carbonaceous stratum and the resulting combustion zone is caused to movethrough the stratum by either inverse or direct air drive whereby the heat of combustionof a substantial proportion of the hydrocarbon in the stratum drives out and, in the case of inverse ,drive, upgrades a substantial proportion of the unburned hydrocarbon material.

The ignition, of carbonaceous material in a stratum around a borehole therein, followed by injection of air through the ignition borehole in the stratum, constitutes a direct air drive process for eifecting in situ combustion' and recovery of hydrocarbons from the stratum. In this type of operation the stratum frequently plugs infront' of the combustion zone because a heavy viscous liquid bank of hydrocarbon collects in the stratum in advance of the combustion zone which prevents movement of air to the combustion process. To overcome this difliculty and to permit the continued progress of the combustion zone through the stratum, inverse air injection has been resorted to. By this technique, a combustion zone is established around an ignition borehole by any suitable means and air is fed thru the stratum to the combustionresidue is hot because of the high temperature combos tion wave passing thru during the recovery process at which. time the temperature of thestratum reaches the range of about 1100 to 1700 F. Tests on tar sands indicate that the coke residue amounts to about 3 weight percent of the stratum. Hence, recovery of this valuable fuel in usable form is highly desirable. This invention is concerned with the recovery of this residual coke in the form of a combustible fuel gas.

Tar sand strata of a thickness in the range of 200 to 400 feet thick are frequently found. The simple applica tion of inverse burning to strata of such thickness requires either a tremendous compressor capacity or extremely closely spaced wells to provide an adequate air velocity to bring the entire formation thickness into production by in situ combustion in a single burning step. Burning such a stratum in vertical spaced horizontal sections of to 35 feet in thickness would solve this prob:

lem. In strata that contain horizontal layers of improduc'ed by inverse permeable rock or shale was to naturally divide the thick stratum into separate relatively thin horizontal layers, it is relatively simple to separately produce the individual layers by inverse burning. However, a hoe mogeneous stratum presents a special problem if it is to be produced with reasonable compressor capacity and with reasonable well spacing. The instant invention is concerned with a solution to this problem.

Accordingly, it is an object of the invention to provide a process for producing a relatively thick relatively homogeneous carbonaceous stratum in successive layers with ordinary injection pressure and ordinary well spacing.

Another object is to provide a process for directing the. advance of an inverse burning combustion front alonga selected path at a selected level in a stratum. Other] objects of the invention will become apparent upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises packing off a section of a production borehole at a selected level in a relatively thick homogeneous stratum; initiating and establishing a combustion zone around the packed-off 'section; injecting air, or a premix of air and fuel gas,

thru an injection borehole so as to force the gas to the combustion zone and cause a combustion front to move out from the packed-off section of the production borehole toward the injection borehole until the inverse burning front has progressed a substantial distance into the stratum and has begun to expand above and below the level defined by the packed-off area; at this point, in-

jecting a suitable transport gas thru the packed-off sec-. tion into the burned area of the stratum so as to drivev fluid hydrocarbon material from the heated stratum into the cooler adjacent stratum, thereby forming a viscous bank of hydrocarbon material in the stratum above. and

below the burned area; and thereafter discontinuing the injection of transport gas and renewing the injection of combustion-supporting gas thru the injection well so as to again advance the combustion front towardjthe injection well without substantial vertical expansion of the. combustion zone, particularly adjacentthe burned area existing'at the time of injection of the transport gas.

This cycle of injection of combustion-supporting gas 01:

lowed by injection of transport gas is, repeated periodically was to drive the combustion front by, inverse injection to the injection well. Preferred practice ineludes packing off a corresponding section of the injec- "tion well so as to more accurately control the path of" the inverse combustion front. The injection of combus- I tion supporting gas may be terminated,, reduced, or

continued at regular rates during the injection of transport gas but is preferably reduced or terminated.

Of course, it is to be understood that the process is I normally practiced in a 5, 7, or 9 spot well pattern with radial movement of the .combustion front from the production well to the ring of injection wells surrounding the productionwell. It is also feasible to utilize the process in a line-drive system wherein a line of production wells is flanked on each side by a generally parallel line of injection wells. It isdesirableto effect the first traversal of the stratum by the combustion front at either the uppermost or lowermost level in the stratum and thereafter repeat the process at successively lower or higher levels, respectively. In this manner, the blockedor plugged boundary of a burned-out section of stratum serves as a block to the expansion of the adjacent combustion front into the burned-out area with loss gasf pressure to the burned out area.

The initial combustion zone is established in conventional manner, such as by burning a bed of charcoal or: heavy liquid fuel in' a bed of porous material packed in the production borehole at the selected level so as to heat the adjacent stratum to a temperature above the ignition point of the stratum, and thereafter injecting a premix of air and fuel gas thru the injection wells so that the premix passes to the hot stratum around the production borehole igniting the same and advancing the resulting combustion front thru the stratum toward the injection well. Other methods known in the art may be utilized in initiating combustion around the production well and will not be discussed further.

The transport gas may be air, combustion gases, steam, nitrogen, or any other cheap gas which can be utilized to drive heated fluids into the upper and lower adjacent stratum relative to the burned or combustion area so as to form a heavy liquid bank of hydrocarbon material, such as tar, in the stratum. However, air is the preferred transport gas because this gas burns some of the coke deposit in the paritally burned combustion area thereby heating and driving fluid hydrocarbons into the stratum above and below the combustion area so as to seal off the same to gas flow. It is important to not continue the injection of transport gas to the point where the leading edge of the combustion front is also blocked. Naturally, the first phase of the injection of transport gas affects the boundaries of the combustion zone or burnedout area nearer the production well before the area more remote thereto and, hence, the upper and lower boundaries of the burned area can be preferentially closed to gas flow thereby leaving the outermost edge of the combustion area unplugged so that injection of combustion-supporting gas thru the injection wells can readily pass thru the leading edge of the combustion zone and support combustion therein, thereby advancing the combustion front as desired.

A more complete understanding of the invention may be had by reference to the accompanying drawing which is an elevation in partial section thru a thick carbonaceous stratum showing an arrangement of apparatus for effecting the invention.

Referring to the drawing, a stratum is penetrated by a production well 12 and an injection well 14. A section of well 12, designated 16, is packed off by means of packers l8 and 19. Production and injection tubing extends thru packer 18 to section 16. Casing 22 in well 12 extends substantially to the upper level of stratum 10 and is provided with a valved-conduit 24.

A section 26 of well 14 is packed off at a corresponding level to section 16 by means of packers 27 and 28. Well 14 is provided with a casing 30 and tubing 32 which communicates with section 26. At the stage of the process illustrated, burned area 34 has been formed around well 12 adjacent packed off section 16 extending outwardly toward a ring of injection wells 14, one of which is shown. Transport gas has been injected thru tubing 20 into section 16 so as to form a block of viscous hydrocarbon material 36 above and below burned area 34 and injection of combustion supporting gas thru tubing 32 has been renewed so as to further advance the combustion front at 38 and to produce hydrocarbons thru tubing 20.

This alternate injection of air (or premix) and transport gas is continued until the combustion front has been advanced to the injection well and, thereafter, another inverse burning combustion front is advanced thru another selected layer or level of the stratum.

The time of air injection pulses and frequency neces sary to maintain the top and bottom of the burned area blocked to gas flow depends upon the type of stratum being produced and can readily be found by gradually increasing the time and frequency until the desired amount of blocking is obtained. The increase in injection pressure of the transport gas is a good measure of the amount of blocking being effected and can be utilized as an indicator or when to terminate the injection of blocking gas and resume the injection of air or premix. In the early stages of the process in the average stratum, the transport gas is injected for periods of 2 or S minutes to 10 or 15 minutes every 6 to 12 hours with the injection of combustion-supporting gas during the remainder of the cycle. As the burning process progresses the length of injection period may be increased appreciably.

In the drawing, only one injection Well 14 is shown, but it should be understood that this well may represent a ring of injection wells around central production well 12. In this type of drive, combustion zone 34 expands radially from Well 12 toward all of the injection wells. Well 12 may also be considered one of a line of produc tion Wells flanked on each side by a line of injection wells 14, in a line drive pattern or process.

An aspect of the invention comprises opening the production casing 22 to flow thru conduit 24 during injection of transport gas to allow a controlled bleed-off of the gas around the casing so as to effect more rapid blocking of the stratum near the production well. In instances where the bleed-off of injected air or other transport gas is appreciable, the same may be recycled to the compressor delivering transport gas to tubing 26.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for driving an inverse combustion front thru a relatively thin section of a relatively thick carbonaceous stratum from a selected level in a production Well to a selected level in an injection Well, comprising moving a combustion front from said level in said production well into said stratum toward said injection Well by injecting a combustion-supporting free-oxygencontaining gas into said stratum at said level in said injection well to feed said front; after said front has progressed into said stratum a substantial distance, injecting a transport gas into the partially burned out area of stratum behind said combustion front so as to transport fluid hydrocarbons into cold unburned stratum above and below said area, thereby causing a liquid bank of hydrocarbon material to block flow of gas into the stratum above and below said area; terminating the flow of said transport gas before plugging of said stratum adjacent the leading edge of said combustion front occurs; thereafter, while the leading edge of said combustion front is at combustion supporting temperature, again feeding said combustion-supporting gas to said area from said injection well to advance said combustion front; periodically repeating the preceding cycle of plugging said stratum and advancing said front; and recovering produced hydrocarbons and combustion gases thru said production well.

2. The process of claim 1 wherein said transport gas comprises air.

3. The process of claim 1 wherein said transport gas comprises combustion gases.

4. The process of claim 1 wherein said transport gas comprises steam.

5. The process of claim 1 wherein said recovered produced hydrocarbons and combustion gases are separated and the latter are injected as said transport gas.

6. The process of claim 1 wherein a minor proportion of a combustible gas suflicient to aid in re-establishing combustion is included in the injected combustionsupporting gas.

7. The process of claim l wherein the selected sections of the stratum at said levels are packed off in each well and tubing is extended into the packed off sections; and including the step of opening the production annulus around said tubing to flow during an early phase of injection of transport gas into said burned out area, so as to allow gas to escape to said annulus from said area and enhance the plugging of said stratum adjacent said production well.

8. The process of claim 1 including the steps of driving said front to said injection well; and driving at least one additional front thru at least one additional thin section at at least one additional level in said stratum.

9. The process of claim 1 in which the flow of said combustion-supporting gas is reduced during injection of said transport gas.

10. The process of claim 1 in which the flow of said combustion-supporting gas is terminated during injection of said transport gas.

6 References Cited in the file of this patent UNITED STATES PATENTS 2,793,696 Morse May 28, 1957 2,862,557 Utenhove et a1. Dec. 2, 1958 2,874,777 Tadema Feb. 24, 1959 2,880,803 Parker Apr. 7, 1959 

